Nitride semiconductors such as gallium nitride (GaN), aluminum gallium nitride (AlGaN) and indium gallium nitride (InGaN) have attracted attention as materials for light-emitting elements which can emit light from red to UV.
One of method for growing nitride semiconductor crystal is the hydride vapor phase epitaxy (HYPE) method using a metal chloride gas and ammonia as raw materials. The crystal growth rate when using the HYPE method ranges from not less than 10 μm/h to not less than 100 μm/h and is much higher than the crystal growth rate when using the metal-organic vapor phase epitaxy (MOVPE) method or the molecular beam epitaxy (MBE) method which is typically several μm/h.
For this reason, the HYPE method is often used to manufacture, e.g., GaN free-standing substrate (see, e.g., PTL 1) and MN free-standing substrate. The free-standing substrate here means a substrate having strength which is enough to maintain its shape but does not cause any inconvenience in handling.
Meanwhile, light-emitting diode (LED) formed of a nitride semiconductor is generally formed on a sapphire substrate. For crystal growth in this case, a buffer layer is formed on a surface of the substrate, and then, a thick GaN layer of about 10 to 15 μm including an n-type cladding layer, an InGaN/GaN multiple quantum well light-emitting layer (total thickness of several hundred nm) and a p-type cladding layer (thickness of 200 to 500 nm) are formed in this order on the buffer layer. The reason why the GaN layer located under the light-emitting layer is thick is that GaN on the sapphire substrate can have high crystallinity.
When crystal is grown by the MOVPE method, time required to form such LED structure is typically about 6 hours, about half of which is spent to grow the GaN layer, called template, located under the light-emitting layer. Considering this fact, if the HYPE method with significantly high growth rate can be used to grow a template portion, it is possible to significantly reduce growth time, leading to drastic reduction in the LED wafer manufacturing cost.
Meanwhile, when manufacturing nitride semiconductor template, there is a problem of warpage caused by a large difference in thermal expansion coefficient between sapphire and nitride semiconductor layer, regardless of the crystal growth method. For example, in case that a GaN layer having a thickness of about 10 to 15 μm is grown on a sapphire substrate having a general size, warpage of about 120 to 180 μm occurs when the sapphire substrate has a diameter of 50.8 mm and a thickness of 430 μm±15 μm, warpage of about 260 to 400 μm occurs when the sapphire substrate has a diameter of 100 mm and a thickness of 650 μm±20 μm, and warpage of about 160 to 240 μm occurs when the sapphire substrate has a diameter of 150 mm and a thickness of 1300 μm±25 μm.
To reduce the amount of warpage, the thickness of the sapphire substrate could be increased or the thickness of the nitride semiconductor layer could be reduced. However, theses cause problems that the cost increases when increasing the thickness of the sapphire substrate and crystal quality decreases when reducing the thickness of the nitride semiconductor layer.
Other than those, a method in which a stress-offset layer is formed on the back surface of the substrate (see, e.g., PTL 2) and a method in which a stress relaxation layer is formed on the back and side surfaces of the substrate (see, e.g., PTL 3) are also known as the methods of reducing warpage of nitride semiconductor template.